Information on geometry of a road on which a vehicle is traveling may be employed to assist the vehicle's cruise either automatically or semi-automatically in a similar way to an ACC (Adaptive Cruise Control) system. For example, map information of a car navigation system mounted on the vehicle may be employed for recognition of geometry of the road on which the vehicle is traveling. However, since the map information of the car navigation system is typically recorded in a recording device installed on the vehicle, the information not necessarily reflects current status of the road. Also, since position detection accuracy of a GPS (Global Position System) employed in the car navigation system is relatively coarse as compared to width and curving geometry of the road, it is hard to say that the information is sufficient for cruise assistance.
In view of the foregoing, there have been some techniques developed that employ a radar device mounted on a traveling vehicle to detect geometry of a road on which the vehicle is traveling and employ information on the geometry of the road for cruise assistance. For example, as described in Patent Document 1, from among a plurality of points that are represented in a coordinate system based on factors such as distances and orientations of reflecting objects obtained as results of detections made by radio wave radars, a group of points that are close to each another are extracted, and if a length of a line that connects the points within this group is greater than or equal to a certain length, then the points are deemed to be reflecting targets that exist on a roadside and thus are recognized as the geometry of the road on which the vehicle is traveling.